The invention relates to optical beacon systems that implement communication using optical signals, and more particularly to an improved optical head structure installed within an automobile.
An optical beacon is a type of communication system between automobiles and roads using rays of light in the near infrared region. The optical beacon is also a bidirectional communication system that transmits traffic information such as traffic jam and parking area information from the road side to the automobile side and that transmits IDs specific to individual automobiles and the like from the automobile side to the road side.
While beacons include radio beacons using electric waves, optical beacons use light as their transmission medium and space propagation light as their carrier wave, and implements communication by modulating such carrier wave.
An optical beacon uses a light emitting element and a light receiving element as elements in place of an antenna, and receives optical signals from the road side by the light receiving element and transmits optical signals from the automobile side to the road side by the light emitting element. Incidentally, the wavelength of light used is in the near infrared region around 850 nm. While a combination of the light emitting element and the light receiving element or a collective body consisting of a light emitting element and a light receiving element is called an optical beacon antenna, they will hereinafter be referred to simply as the "optical beacon".
By the way, this optical beacon has the basic function of receiving light and emitting light. Therefore, when an optical beacon is enclosed with a body, a transmission window must be formed in the body in order to ensure transmission of light. FIG. 9 is a schematic diagram showing a body 13 of an optical beacon 10 having such a transmission window 14.
FIG. 9 shows a conventional example of an optical head 10 of an optical beacon (hereinafter referred to simply as the "optical head 10") used for automobiles. As shown in FIG. 9, this optical head 10 has a light emitting section 11 and a light receiving section 12 mounted on a single board 3. The light emitting section 11 has an array of light emitting elements 1 that are constructed of LEDs (light-emitting diodes), and the light receiving section 12 has a large area light receiving element 2 constructed of a PD (photodiode). In FIG. 9, reference numeral 13 denotes a body and 14, a transmission window for transmission and reception of light.
As shown in FIG. 10, this mounting board 3 is disposed so as to be inclined at an angle corresponding to an optimal directive angle of the optical head 10 with respect to the upper surface of a substantially horizontal installation surface within an automobile, e.g., the upper surface of an instrument panel so that the transmission and reception efficiency of the light emitting elements 1 and the light receiving element 2 (hereinafter referred to simply as the "optical element" collectively whenever applicable) is optimized. In general, the optimal directive angle of the optical head 10 is set to, e.g., 43.degree..+-.10.degree. with respect to a horizontal plane.
Further, the reason why the LEDs are arrayed is not only to increase the sum total of energy but also to make a fine adjustment of the directive angle as a whole by individually setting the mounting angles of the respective LEDs as shown in FIG. 10. In FIG. 10, the arrows indicate light emitting directions.
While the light emitting elements 1 and the light receiving element 2 are arranged on a single board 3 in the example shown in FIG. 9, they may not necessarily be arranged in such a manner. The light emitting elements 1 and the light receiving element 2 may be arranged in any mode as long as such mode allows rays of light to be transmitted and received through the transmission window 14 formed in a surface 21 of the body 13 confronting the windshield glass surface when the light emitting elements 1 and the light receiving element 2 are accommodated in the body 13 such as shown in FIG. 10.
A road-side optical beacon unit 52 and an automobile-side optical beacon unit 51 are arranged as shown in FIG. 11, respectively, and the directive angle of the antenna of the automobile-side unit 51 is required to be set to 43.degree..+-.10.degree. with respect to a horizontal plane. If the body 13 of the optical beacon 10 is to be arranged inside the automobile room, the optical beacon 10 can most advantageously keep this directive angle when placed on the upper surface of an instrument panel 23 that confronts a windshield glass 20 such as shown in FIG. 12. Almost all the body 13 of the optical beacons 10 are disposed on the upper surface of the instrument panel 23 that is substantially horizontal.
In this case, a problem arises. That is, when an additionally arranged part such as the body 13 is disposed on the upper surface of the instrument panel 23, a reflected image a'b' of (the surface 21 of) the body 13 is mirrored on the windshield glass 20 to thereby disturb the view of a driver D. As generally taken measures to this problem, light is scattered by, e.g., embossing the surface of the body 13 in order to control the reflectance of the surface of the body 13, or the surface pattern of the body 13 is made analogous to that of the instrument panel 23 in order to give continuity to the instrument panel 23, or the number of partings is reduced, etc.
However, since the region occupied by the transmission window 14 for light transmits light, no surface treatments such as described above can be given to this region. Therefore, a reflected image of this region is, in any way, formed on the windshield glass 20 to thereby disturb the view of the driver D.
Furthermore, the above-mentioned conventional optical head 10 has addressed the following problems.
While the mounting board 3 of the optical element must be mounted so as to be inclined with respect to the horizontal surface as described above, the control board 4 and the mounting board 3 must be arranged under different layout systems spacewise as shown in FIGS. 9 and 10, taking into consideration ease with which to operate the control board for controlling the optical element. In this case, the control board 4 and the mounting board 3 are usually formed to be separate pieces.
If these boards 3, 4 are formed into separate pieces, connecting means for connecting these boards 3, 4 mechanically and electrically must be provided separately as well, which in turn complicates the general configuration of the optical head 10 and addresses manufacturing and managing problems.
The arrangement in which the LEDs are arrayed with each LED being inclined at a certain angle addresses a serious problem that the number of manufacturing steps is increased and that management is complicated. Further, if the LEDs must be rearrayed for adjustment of their directive angles after mounted, the LEDs may likely be mechanically deformed due to vibrations or the like during the operation or may likely break connecting electric wires.
Since the PD with a large area not only is expensive but also has a large capacity, its speed of response is low. Further, shot noise increases with increasing area, which in turn impairs S/N ratio.